This invention relates to PEM/SPE fuel cells, and more particularly, to a method of starting-up such fuel cells from subfreezing temperatures.
Fuel cells have been proposed as a power source for many applications. So-called PEM (proton exchange membrane) fuel cells [a.k.a. SPE (solid polymer electrolyte) fuel cells] are particularly desirable for both mobile (e.g. electric vehicles) and stationary applications. PEM/SPE fuel cells include a xe2x80x9cmembrane electrode assemblyxe2x80x9d (hereafter. MEA) comprising a thin proton-conductive (i.e. H+-conductive), solid-polymer, membrane-electrolyte having an anode on one of its faces and a cathode on the opposite face. The solid-polymer membranes are typically made from ion exchange resins such as perfluoronated sulfonic acid. One such resin is NAFION(trademark) sold by the DuPont Company. Such membranes are well known in the art and are described in U.S. Pat. No. 5,272,017 and 3,134,697 as well as in the Journal of Power sources, Vol. 29, (1990), pages 367-387, inter alia. The anode and cathode typically comprise finely divided catalytic particles either alone or supported on the internal and external surfaces of carbon particles, and have proton conductive resin intermingled therewith. The anode and cathode catalysts cover opposite faces of a solid polymer membrane electrolyte.
The MEA is sandwiched between a pair of electrically conductive current collectors for the anode and cathode. The current collectors contain channels/grooves on the faces thereof defining a xe2x80x9cflow fieldxe2x80x9d for distributing the fuel cell""s gaseous reactants (i.e. H2 and O2) over the surfaces of the respective anode and cathode catalysts. Hydrogen is the anode reactant (i.e. fuel) and can either be in a pure form or derived from the reformation of methanol, gasoline or the like. Oxygen is the cathode reactant (i.e. oxidant), and can be either in a pure form or diluted with nitrogen (e.g. air). The overall electrochemical reaction occurring at the MEA under normal fuel cell operation is: (1) H2 is oxidized on the anode catalyst to form 2H+ ions and releases 2 electrons to the external circuit; (2) the H+ ions move through the membrane to its cathode side; (3) the 2 electrons flow through the external circuit to the cathode side of the membrane where they reduce the O2 on the cathode catalyst to form Oxe2x88x92 ions; and (4) The Oxe2x88x92 ions react with the H+ ions on the cathode side of the membrane to form water.
It is desirable for many applications, and particularly electric vehicle applications (i.e. to meet customer expectations), that the fuel cell be capable of being started-up quickly so as to be immediately available to produce the energy needed to propel the vehicle without significant delay. At high ambient temperatures (e.g. about 20xc2x0 C. or more), the fuel cell stack (i.e. plurality of individual cells bundled together into a high voltage pack) can be started-up in a reasonable amount of time because electrical current can be immediately drawn from the stack which, in turn, causes electrical IR-heating of the stack to quickly heat up the stack to its preferred operating temperature (i.e. about 80xc2x0 C.). At subfreezing temperatures below about xe2x88x9225xc2x0 C., however, rapid start-up is much more difficult, because at these temperatures the rate at which the overall electrochemical reaction occurring at the membrane-electrode-assembly takes place is significantly reduced thereby limiting the amount of current that can be drawn from the stack, and hence the IR-heating that can be inputted to the stack. The precise mechanism for the reaction rate reduction is not known. However, it is believed to be that either (1) the H+ ion conductivity of the solid polymer membrane electrolytes is so poor at these temperatures, (2) or the effectiveness of the catalysts to electrochemically ionize the H2 and/or O2 is so poor at these temperatures, that no significant amount of electrical current can be drawn from the stack, and no corresponding IR-heating thereof can occur.
The present invention comprehends a method of heating the MEA of a PEM fuel cell while it is cold to thaw it out and thereby accelerate cold start-up of the fuel cell. The method applies to single cells as well as a stack of such cells. The fuel cell has a MEA that comprises a proton-conductive membrane, a cathode catalyst supported on a first face of the membrane, and an anode catalyst supported on a second face of the membrane opposite the first face. In accordance with the present invention, the MEA is thawed out by locally heating it using the heat generated by the exothermal chemical reaction between H2 and O2 on the anode and/or cathode catalyst(s) which raises the MEA""s temperature from a first subfreezing temperature to a second temperature which is above the first temperature and which enhances the rate of the overall electrochemical reaction occurring at the MEA. More specifically, the method of the present invention comprises the steps of: (1) supplying a H2-rich gas (e.g. pure H2 or CO-containing reformate) to the anode catalyst and a O2-rich gas (e.g. pure O2 or air) to the cathode catalyst; (2) introducing a sufficient quantity of H2 into the O2-rich gas, and/or a sufficient quantity of O2 into the H2-rich gas to exothermally chemically react the H2 with the O2, and thereby assist in heating the MEA up to a second temperature where current can be drawn from the fuel cell; (3) discontinuing the introduction of such quantities of H2 and/or O2 after the MEA reaches a suitable temperature at or above the second temperature; and (4) drawing electrical current from the fuel cell to assist in completing the heating of the fuel cell up to its normal operating temperature.
The H2-rich gas that fuels the anode may be the source of the H2 provided to the O2-rich cathode gas, and air may be the source of the O2 provided to the H2-rich gas. The amount of H2 introduced into the O2-rich gas is such as would produce a mix having a hydrogen content of about 0.5% to about 3.5% by volume. O2 concentrations as low as about 1% and as high as 7% by volume (i.e. when mixed with the H2-rich gas) can be used when pure H2 is the fuel. When CO-containing H2-rich gases (e.g. reformate) are used, O2 concentrations between 2% and about 7% by volume are preferred.
The faces of the membrane that support the catalysts each has (1) a leading edge that first contacts the O2-rich/H2-rich gas (es), and (2) a trailing edge that last contacts O2-rich/H2-rich gases as the gas (es) flow over the appropriate cathode or anode face. During the thawing step of this invention, the O2-rich and/or H2-rich gases may be flowed across their associated MEA faces from the leading edge toward the trailing edge at a flow rate greater that than the flow rate used for the normal operation of the fuel cell once it has reached its normal operating temperature. This higher rate insures that much of the O2 and/or H2, as appropriate, is/are swept downstream of the leading edge to react on catalyst downstream of the leading edge so as to heat the MEA more evenly than would occur if the O2/H2 gas were flowed at a slower rate and mostly reacted near the leading edge. In this regard, slow flow rates tends to increase the residence time of the O2/H2 near the leading edge which causes more of the O2/H2 to react thereat causing uneven heating of the MEA. Hence by way of example, if the flow rate of H2 through a given stack during normal operations were 0.01 kg/min., a useful flow rate during MEA thawing might be about 0.04 kg/min. Similarly, if the flow rate of O2 through a given stack during normal operations were 0.16 kg/min., a useful flow rate during MEA thawing might be about 0.64 kg/min. Alternatively, the gas flow channels through which the O2-rich and H2-rich gases flow could be configured (e.g. tapered) such that the gas velocity therein changes from a first higher velocity at the leading edge to a second lower velocity downstream of the leading edge which will also serve to effect more even heating of the MEA.
If the fuel cell stack were shut down xe2x80x9cwetxe2x80x9d (i.e. with free water present), ice could form atop the catalyst(s) when the stack is subjected to freezing temperatures. Such ice could block access to the catalyst(s) by the H2 and/or O2 and prevent the desired chemical reaction from occurring. In the event of such icing, it is preferred to warm (i.e. above the temperature of the MEA) dry H2-rich and/or O2-rich gas (es) and pass it/them through the associated flow field(s) for a sufficient time to de-ice the catalyst(s) before introducing the O2 and/or H2 into the fuel or oxidant streams.
The present invention will better be understood when considered in the light of the following detailed description of one embodiment thereof which is given hereafter in conjunction with the Figure in which: